The development of low loss, high silica lightguide fiber over the last few years has led to the investigation of high temperature (e.g., approximately 2000.degree. C.) heat sources, for the drawing of high strength fiber from a lightguide preform. Of the possible heat sources, the oxy-hydrogen torch, the CO.sub.2 laser and a few induction and resistance furnaces have been employed for drawing the high silica fibers. The torch method, while inexpensive, cannot maintain a uniform diameter over long lengths of drawn fiber. The CO.sub.2 laser provides the cleanest drawing atmosphere, but requires special optical designs to radially distribute the energy for drawing and is limited in power, while resistance furnaces require an inert protective atmosphere to prevent oxidation of the heating element.
An induction furnace is basically comprised of a hollow, centrally located, tubular susceptor surrounded by an insulating material. An induction coil surrounds the insulating material to provide an alternating electromagnetic field when energized. The field couples to the susceptor, elevating the temperature thereof, forming a hot zone therein. A glass lightguide preform is then introduced into the hot zone to reflow a portion thereof from which the lightguide fiber is drawn.
High temperature induction furnaces provide high thermal inertia, stability, and radially symmetric heating. Most induction furnaces, however, use graphite or refractory metallic susceptors which require the flowing of protective atmospheres during operation to remove contaminants migrating from the inside surface of the tube. As a result, such furnaces have limited susceptor life and have some degree of contamination in the furnace atmosphere.
One induction furnace designed to overcome the foregoing problems is described in an article entitled "A Zirconia Induction Furnace For Drawing Precision Silica Wave Guides" by R. B. Runk in the Optical Fiber Transmission II technical digest (Tu B5-1), Feb. 22-24, 1977 which is incorporated by reference herein. That furnace uses a cylindrical susceptor made of yttria stabilized zirconium dioxide. The susceptor has a long life expectancy and minimal furnace atmosphere contamination in an oxygen-bearing atmosphere. Such a furnace has been shown to be most effective in drawing lightguide fiber from a preform. However, it has been found that after extended use microscopic particles of zirconium dioxide migrate from the susceptor onto the preform and/or the fiber being drawn from the lightguide preform. The zirconium dioxide particles substantially weaken the drawn fiber resulting in unacceptable product.
Accordingly, there is a need for a long life, high temperature induction furnace that is substantially contaminant-free during the drawing process in order to produce high strength fibers.